Log entry time 18:13:30 on July 24, 2005

Entry number 149308

This entry is a followup to: 149306

Target density "tweak"

We are at a target temperature and presure such that we would not be able to run much over the 35 µA that we have been limited to by the septum. Given that we hope to gain up to about 5 µA by running the septum at higher current, I decide we need to tweak the target. Running the cell temp up a bit improves the cooling efficiency as it makes a significant difference in the delta T between the coolant and the gas in the target.

First, measure the target parameters vs. cell temperature:

  Htr_aft   HTR_fore  Cell_aft  Pressure   CHL   HighPowerHeater
(cell temp)   (K)      (K)        (psi)   (g/s)      (W)
---------   -------   -------   --------  -----  --------------
  6.41       6.47      6.51      202.4     22.2     315
  6.49       6.55      6.62      208.7     22.0     325
  6.60       6.66      6.72      214.2     22.0     336
  6.69       6.74      6.82      220.0     21.9     350

  6.30       6.37      6.41      196.0     22.0     300
  6.19       6.27      6.31      190.0     21.8     270
  6.10       6.16      6.21      184.0     21.8     260
  6.00       6.07      6.11      178.0     21.8     240 

Returned cell to 6.4 K, 202.4 psi.

Now, during a controlled access, we bleed off a wee bit of gas to get to 190 psi at 6.4 K; this corresponds to a reduction in density of from 0.136 g/c down to 0.134 g/cc, i.e. a 1.5% reduction.

  Htr_aft   HTR_fore  Cell_aft  Pressure   CHL   HighPowerHeater
(cell temp)   (K)      (K)        (psi)   (g/s)      (W)
---------   -------   -------   --------  -----  --------------
  6.40       6.47      6.50       190.0    21.9    312
  6.50       6.56      6.61       195.0    21.9    335
  6.60       6.66      6.69       201.0    21.9    340
  6.70       6.76      6.80       207.2    21.9    360  

At this new operating point (6.7 K, 207 psi) we are still reasonably far away from the 230 psi nominal vent point (yes, I know, it didn't vent there in practice). However, if there are large CHL flow variations causing big temperature and pressure excursions, we could lower the cell temp again.

We have gained 45 W in cooling power with this exercise, which corresponds to 8 µA of beam current (note: 8 µA is a 22% increase over 35 µA, so the 1.5% density loss is negligible in comparison).

If we find that we can get more beam current we can bleed off a bit more gas and continue the process. Last year we took a test where we went up to 7.6 K operating point, and the cooling efficiency continued to improve.

I doubt this small tweak will affect the target density fluctuations at all.